I got a Gibson question for you..About a Les Paul Custom

I was at GC today at lunch just browsing and came across a 70-something Les Paul Custom, I believe a 78. This thing was a pearl pinkish/red color that I had never seen before. It was also the "thin" ones that they made a few of. Now here is my question.

The serial number had a "sec" stamped above it. Was this a "second" or something? Or is "sec" another term for something else?